Guideline Document-8

View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Loughborough University Institutional Repository This item was submitted to Loughborough’s Institutional Repository (https://dspace.lboro.ac.uk/) by the author and is made available under the following Creative Commons Licence conditions. For the full text of this licence, please go to: http://creativecommons.org/licenses/by-nc-nd/2.5/ Construction and Repair with Wet-Process Sprayed Concrete and Mortar Construction and Repair with Wet-Process Sprayed Concrete and Mortar S.A.Austin1, P.J.Robins2 and C.I.Goodier3 1. Professor of Structural Engineering 2. Senior Lecturer 3. Research Associate Department of Civil and Building Engineering at Loughborough University, Loughborough, Leics, LE11 3TU. Contact: S.A.Austin: email : [email protected] Tel: + 44(0)1509 222608 Fax: + 44(0)1509 223981 Construction and Repair with Wet-Process Sprayed Concrete and Mortar Loughborough University 1 Introduction This document provides practical guidance for designers, specifiers, contractors and 1clients on all aspects of low- volume wet-process sprayed mortars and concretes. It provides information for both new construction and small- scale repair and covers choice of application method, materials and mixes, specification, pumping and spraying, finishing, curing, testing and performance. The information is a combination of existing good practice and new knowledge acquired during a three-year research project conducted at Loughborough University entitled ‘Wet Process Sprayed Concrete for Repair’. This was funded by both Government1 and industry, namely Balvac Whitley Moran, Fibre Technology, Fosroc International, Gunform International Ltd and Putzmeister UK Ltd. This document concentrates on wet-process mortars and small aggregate concretes (< 8 mm) applied in thin layers (<100 mm) at low/medium output rates (< 5m3/hr), in some cases with mesh or fibre reinforcement. Wet- and dry-process sprayed concrete and mortar has been described by several names in the past, namely shotcrete and gunite. This document uses the terminology standardised by the European Federation of National Associations of Specialist Repair Contractors (EFNARC), namely sprayed concrete, with mixes containing aggregate with a maximum size of 3-4 mm being classed as mortars and anything larger as concretes. Sprayed concrete can be defined as ‘mortar or concrete conveyed through a hose and pneumatically projected at high velocity from a nozzle into place.’ In the wet process, the constituents (cement, aggregate, admixtures and water) are batched and mixed together before being fed into the delivery equipment or pump. The mix is then conveyed under pressure to the nozzle, where compressed air is injected to project the mix into place. This differs from the dry process in which the dry constituents are batched together before being conveyed under pressure through the delivery hose to the nozzle, where pressurised water is introduced and the mix projected into place. Sprayed concrete and mortar can offer a number of advantages over cast in-situ concrete including: • reduction or elimination of formwork (with consequent cost savings) • construction of free-form profiles • rapid placement of large volumes • the production of dense, homogenous, high quality concrete • reduced access problems (by locating the delivery/mixing equipment away from the placing area) • good bond to substrate and between layers • reduced thermal stresses (by placing several thinner layers) • production of unusual finishes. Disadvantages associated with sprayed concrete and mortar include: • specialist expertise needed for both good design and construction • variable concrete quality (though mainly with the dry process) • high material costs (arising from specialist mixes, high cement contents and wastage due to rebound, overspray and cutback) • poor encasement behind dense concentrations of reinforcement • labour-intensive finishing • effort required in quality control testing to ensure a satisfactory finished product. 1 Funded by the Engineering and Physical Science Research Council (EPSRC), Grant number GR/K52829 2 Construction and Repair with Wet-Process Sprayed Concrete and Mortar Loughborough University 2 Applications of Sprayed Concrete Many aspects of preparation and construction for sprayed concrete apply equally to new construction and to repair. For both applications it is advisable to seek the views of contractors experienced in sprayed concrete who can provide guidance on the practical aspects of the work and identify any requirements in the draft specification that may cause problems. 2.1 New construction Conventionally placed concrete is generally cheaper than sprayed concrete but spraying enables the creation of free- formed shapes. Simple domes or hyperbolic parabaloid roofs can be conventionally cast, but they are more rapidly and economically produced by spraying. The as-shot surface can be an architectural feature or thin flash coats can produce various types of finish. Spraying is a popular method of constructing swimming pools, especially modern leisure pools which are frequently complex in shape and other hydraulic structures (Figure 1). Sprayed and cast concrete can be combined e.g. in swimming pools where the base is cast and the walls are sprayed. Sprayed concrete can also provide fire protection for steel frames. The Shanghai Bank in Hong Kong is a high-profile example where 1000m3 of concrete was sprayed over an area of 72,000m2. Canal linings, dams, harbours, sea walls and defences, chimney linings, arches, domes and storage tanks can all be constructed with sprayed concrete. One of the largest new constructions in wet-process sprayed concrete was the New Zoological Gardens in Riyadh, Saudi Arabia, where over 12,500m3 of concrete was sprayed in structures and finishes (Figure 2). Sprayed refractory concretes are utilised in the cement, iron and steel industries for kiln and furnace linings, chimney flues and casting chambers. Sprayed concrete has also been used for sculpture. Rachel Whiteread won the Turner prize in 1993 with a sculpture of the inside of a Victorian terraced house which had been sprayed on the inside with sprayed concrete. The existing structure was then carefully demolished to reveal a negative image of the dwelling. This was completed using the dry-process, but in hindsight the wet-process would have been preferable as the better working environment and reduced rebound would have been more suited to the enclosed conditions within the house. Figure 1.Pierepoint canoe slalom Figure 2. Riyadh zoo, Saudi Arabia 2.2 Underground support Underground support, rock stabilisation and tunnel linings are a major use of wet-process sprayed concrete but they are outside the scope of this document and a large amount of information is already available on these applications (Daws, 1995; ITA, 1993; Melbye, 1995 and Melbye, 1997). When applied to a rock surface, sprayed concrete is forced into fissures and open joints and helps to bond such features together. It also helps prevent water ingress and protects the rock against deterioration by water and air. Rock bolts and steel fibres are widely utilised in rock stabilisation and underground support. An example in the UK of sprayed concrete providing rock stabilisation is the 100 km North Wales A55 coast road completed in 1994 (Figure 3). The New Austrian Tunnelling Method (NATM) also incorporates sprayed concrete to provide temporary support in soft or weak ground and several guides are available on this technique (Barton, 1995 and ICE, 1996). Figure 3. Rhualt Hill 2.3 Concrete repair and strengthening Sprayed concrete repairs have been successfully completed to many structures including: bridge soffits, beams, parapets and abutments; steel and reinforced concrete framed buildings; cathodic protection; cooling towers; industrial chimneys; tunnels; water-retaining structures; jetties, sea walls and other marine structures (Taylor, 1995). 3 Construction and Repair with Wet-Process Sprayed Concrete and Mortar Loughborough University A recent successful example which illustrates the flexibility of wet-process sprayed concrete was the £4.3m contract to strengthen and repair the Lancaster Place Vaults (Figure 4) at the North end of Waterloo bridge in London (Bridge, 1999). For strengthening the brick archways sprayed concrete was cheaper than conventionally cast concrete as the irregular shapes of the arches eliminated the possibility of re-using shutters. 500m3 of concrete incorporating a plasticiser and a stabiliser was sprayed and an activator was added at the nozzle. Figure 4. Brick archway strengthening at Lancaster Place Vaults (before application of sprayed concrete) Causes and types of defects Repairs can be broadly categorised into structural, serviceability and cosmetic repairs. Structural repairs restore lost sectional or monolithic properties to damaged concrete members, serviceability repairs restore concrete surfaces to a satisfactory operational standard, and cosmetic repairs restore concrete to a satisfactory/acceptable appearance. A recent survey by Loughborough University of 11 organisations (consultants, contractors and local authorities) involved with concrete repair in the UK found that the main cause of deterioration of concrete leading to repair is the corrosion of steel reinforcement due to the ingress of chlorides and carbonation, with alkali aggregate reaction and the effects of heat following fires being less

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